Fuel cells which generate electric current by the electrochemical combination of hydrogen and oxygen are well known. In one form of such a fuel cell, an anodic layer and a cathodic layer are separated by an electrolyte formed of a ceramic solid oxide. Such a fuel cell is known in the art as a “solid oxide fuel cell” (SOFC). Hydrogen, either pure or reformed from hydrocarbons, is flowed along the outer surface of the anode and diffuses into the anode. Oxygen, typically from air, is flowed along the outer surface of the cathode and diffuses into the cathode. Each O2 molecule is split and reduced to two O−2 anions catalytically by the cathode. The oxygen anions diffuse through the electrolyte and combine at the anode/electrolyte interface with four hydrogen ions to form two molecules of water. The anode and the cathode are connected externally through a load to complete the circuit whereby four electrons are transferred from the anode to the cathode. When hydrogen is derived by “reforming” hydrocarbons such as gasoline in the presence of limited oxygen, the “reformate” gas includes CO which is converted to CO2 at the anode via an oxidation process similar to that of the hydrogen. Reformed gasoline is a commonly used fuel in automotive fuel cell applications.
A single cell is capable of generating a relatively small voltage and wattage, typically between about 0.5 volt and about 1.0 volt, depending upon load, and less than about 2 watts per cm2 of cell surface. Therefore, in practice it is known to stack together, in electrical series, a plurality of cells. Because each anode and cathode must have a free space for passage of gas over its surface, the cells are separated by perimeter spacers which are selectively vented to permit flow of gas to the anodes and cathodes as desired but which form seals on their axial surfaces to prevent gas leakage from the sides of the stack. The perimeter spacers may include dielectric layers to insulate the interconnects from each other. Adjacent cells are connected electrically by “interconnect” elements in the stack, the outer surfaces of the anodes and cathodes being electrically connected to their respective interconnects by electrical contacts disposed within the gas-flow space, typically by a metallic foam which is readily gas-permeable or by conductive filaments. The outermost, or end, interconnects of the stack define electric terminals, or “current collectors,” which may be connected across a load.
A complete SOFC system typically includes auxiliary subsystems for, among other requirements, generating fuel by reforming hydrocarbons; tempering the reformate fuel and air entering the stack; providing air to the hydrocarbon reformer; providing air to the cathodes for reaction with hydrogen in the fuel cell stack; providing air for cooling the fuel cell stack; providing combustion air to a combustor for tail gas fuel exiting the stack; and providing air to the combustor and the stack.
During normal operation, the fuel cell stack produces waste heat which must be removed to maintain the correct stack temperature. This is typically accomplished by flowing excess cathode air which then provides a cooling function from within the stack. However, this excess cathode air, which may be three to four times the amount required to oxidize the fuel, requires additional power in the air supply system, which power is a parasitic loss and thus reduces the overall efficiency of the system.
At start-up of the system, it is necessary to bring the stack up to its operating temperature of 700-800° C. The time required to bring the stack up to operating temperature is dependent on the temperature and mass flow of the heated cathode air, which in turn is limited by thermal stresses indiced on the fuel cells by the hot air. Thermal modeling has shown that the stack can be heated more quickly if heat is added externally to the stack as well as internally. This is due both to the additional heat being put into the stack and to a reduction in thermal stresses as a result of both internal and external heating.
What is needed is a means for heating the stack externally during start-up to provide faster warm-up, and for cooling the stack externally during normal operation to reduce the volume of cathode air required.
It is a principal object of the present invention to improve the electrical efficiency of a solid-oxide fuel cell system.
It is a further object of the invention to reduce the warm-up period for solid-oxide fuel cell upon start-up.